This invention represents an improvement in devices which utilize ultrasonic waves to detect defects and inhomogeneities in solid materials. In particular, the invention is concerned with the use of surface elastic waves, particularly Rayleigh waves, to examine the near-surface region of a solid material.
Ultrasonic testing of materials has evolved into a major technique for non-destructive testing of materials. One area of ultrasonic testing concerns the examination of the near-surface region of a material for flaws or inhomogeneities through the use of Rayleigh waves as the ultrasonic probe. Rayleigh waves are constrained to travel in the material at the free surface boundaries with the wave penetrating to a depth of approximately one wavelength from the surface. By monitoring changes in the transmitted sound wave spectrum, the nature and extent of flaws or inhomogeneities in a solid may be determined. However, due to the types of materials currently being utilized, the use of Rayleigh waves has been constrained to relatively low frequencies, typically less than 10 MHz with a corresponding lower limit on the wavelength of approximately 300 .mu.m. Acrylic plastic wedges are the predominant, commercially available material used to produce Rayleigh surface waves. The attenuation coefficient is prohibitively large for acoustic modes greater than 10 MHz. As a consequence of this limitation, there is a lower limit on the size of the defect which can be analyzed, and the depth of Rayleigh wave penetration into the solid is so large that limits are placed on the effective layer thickness which can be detected by use of Rayleigh waves. Furthermore, Rayleigh waves of 1-10 MHz frequency have a rather substantial inherent angular divergence upon transmission from the source crystal, thereby limiting the angular sensitivity of an ultrasonic testing device utilizing such low frequency acoustic waves.
Other techniques used to generate Rayleigh waves include the impulse and the comb methods. The impulse technique involves the use of a piezoelectric crystal to apply a stress pulse directly to the test material surface. The stress pulse from the transducer arises by placement of a large, pulsed dc voltage across the transducer. In the comb technique, a series of alternating projections and slots, with a width half a Rayleigh wavelength, are positioned on the test surface and a transducer is placed atop this structure. The transducer is then excited by high voltage pulses. In these different methods, there is an inherent maximum frequency limit of approximately 20 MHz in the impulse method and 50 MHz in the comb method.
It is therefore an object of the invention to provide an ultrasonic testing device using a single crystal metal wedge capable of producing Rayleigh waves to examine the surface region of a test material for flaws and inhomogeneities.
It is a further object of the invention to provide an ultrasonic testing device using a single crystal metal wedge capable of producing Rayleigh waves of very high frequency and corresponding small wavelength to permit fine level resolution of flaws and inhomogeneities.
It is another object of the invention to provide an ultrasonic testing device using a single crystal wedge of metal selected from a particular part of the periodic table and capable of producing Rayleigh waves to examine the surface region of a test material for flaws and inhomogeneities.
It is also an object of the invention to provide an ultrasonic testing device using a single crystal metal wedge capable of transmitting Rayleigh waves with a narrow angle of divergence.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.